The neuropathology of mental retardation is thought to be associated with deficits in synaptic structure and function. Our goal is to make contributions into understanding the formation and maintenance of dendritic spines and of postsynaptic Ca2+ homeostasis in neurons expressing Rett Syndrome-associated mutations in MECP2. Rett syndrome (RTT) is an X- linked developmental disorder and the leading cause of mental retardation in females. Mutations in the transcriptional represser MECP2 have been identified in >90% of RTT cases. One of the target genes of MeCP2 is bdnf, brain-derived neurotrophic factor. Considering that BDNF has recently emerged as a potent modulator of activity-dependent synaptic development and plasticity in the postnatal brain, including fundamental neuronal properties such as dendritic spine density and form and neuronal Ca2+ signaling, we hypothesize that a deregulation of BDNF signaling may underlie the dendritic pathologies observed in RTT. The specific hypothesis to be tested is twofold: 1) RTT-associated MECP2 mutations cause dendritic spine loss leading to Impaired dendritic Ca2+ signaling in hippocampal pyramidal neurons through reduced BDNF signaling; 2) impaired dendritic structure in MECP2 mutant neurons can be reverted by BDNF treatment. The consequences of mutant MECP2 expression will be evaluated in neurons maintained in organotypic slice cultures and transfected by particle-mediated gene-transfer. The biolistic gene-transfer approach provides a more flexible way to introduce different mutant forms of MECP2 compared to the generation of transgenic or knockout mice, in addition to allow the co-transfection of other cDNAs or knockdown siRNA constructs of interest. Thus, it represents a novel cellular model of RTT. Transfected neurons will be studied by laser-scanning confocal and time-lapse multiphoton microscopy, as well as by simultaneous Ca2+ imaging and whole-cell intracellular recordings. This combination of state-of-the-art approaches has never been used to investigate MECP2 function, or applied to animal models of RTT. We expect the proposed studies to provide novel insights into the consequences of mutant MECP2 expression in hippocampal neurons in a cellular model of RTT. [unreadable] [unreadable] [unreadable]